Sunday vs Monday

Why wouldn’t you go to Notre Dame on a Sunday morning? I recommend attending a Sunday service to really experience this awesome cathedral; in fact, I have done this.

In October, I will be in Milan for 5 days and one of those days is a Sunday. I will be going to a Sunday mass at the Duomo (cathedral). I am neither Roman Catholic nor do I speak Italian. I can’t wait to experience their Sunday service. To me, a Sunday service is the essence of experiential travel. The music will be so beautiful as Notre Dame’s services are!

I can't think of any reason why one day would be any better than the other, for the activities you cited. Except that the smaller retail stores in the Marais and Montmartre may be closed on Sunday. Galeries Lafayette are open every day
Re organ recitals at ND: was this what you were looking for? https://musique-sacree-notredamedeparis.fr/categorie/auditions-dorgue-2025-2026/

There's also this: https://musique-sacree-notredamedeparis.fr/ete-2026/#aout

The Marais is an absolute zoo on Sundays. Everything is open, and everyone is there. If there is one place I would (and do) avoid on Sundays, it's the Marais.

St. Sulpice church offers up some pre-mass music on Sundays,before the 11:00 mass. The organ is a renowned one. 10:45 a.m., followed by mass during which the organ will play. Just walk into the church and find a seat. Best not to get up and leave during the service unless you are not sitting in a conspicuous seat, just out of respect.

There are several services throughout the day on Sundays at Notre Dame with music if you are interested. You can quietly tour the Cathedral after the service.

As Kim said. Sundays in the Marais are crazy.

Paris Walks offers a nice walking tour of Montmartre on Sundays that ends at Sacre Coeur.

I would expect Galleries Lafayette to be more crowded on a Sunday. Saturday's can be an absolute madhouse in some departments.